Carlos Fuentes, occasionally dubbed the Mexican Balzac, makes no bones about the grand plans he has for this vast, panoramic novel. "The hell that is Mexico," says one of the characters named Santiago (there are four, they are all related to one another, and they all die miserably). "Are we predestined for crime, violence, corruption, poverty?" Throughout, rhetorical questions, impassioned speeches, fraught dialogues and urgent declarations dominate the book's spoken content; characters are far more likely to debate the heroic or treacherous qualities of Bukharin or the conduct of the Spanish civil war than they are to pass the time of day in idle pleasantries.

But Fuentes is aware that readers need human contact, and thus he gives us Laura Diaz, eye-witness, eavesdropper and occasional participant in the grand march of history - as well as daughter, wife, mother and, in order to give the novel some light entertainment, a rather racy lover of extraordinary stamina.

Laura is a Zelig-like figure who is always close to the heart of the action, but just detached enough to provide some ironic distance. In her youth, she loves and loses her revolutionary half-brother - the first Santiago - to a firing squad; she marries a prominent union leader and harbours a presidential assassin; and enters a raffish, bohemian circle in which she meets a previous Fuentes hero, Artemio Cruz, and the painters Diego Rivera and Frida Kahlo.

The second half of her life sees her engaged in a lengthy affair with a Spanish Republican in exile, find fame as a photographer, and get caught up in McCarthyism. It's hardly a dull life by anyone's standards, although a certain ennui takes its place in Laura's wide-ranging, and not always wholly explicable, emotional life.

On a small scale, Fuentes's subject could be said to be how you become the hero of your own life. For Laura, his first major female protagonist, the answer comes in self-realisation, denoted by the break from her husband Juan Francisco and her late-flowering career. It's noticeable, however, that she always attaches herself to charisma and to powerful personalities; her disillusionment with Juan Francisco is precisely that, and rests largely on her gradual understanding that his show of strength is based on fundamental weakness.

Despite giving full rein to Laura's inner struggles and torments - frequently in the form of stream-of-consciousness passages halfway between speech and thought - Fuentes is far more interested in the grand scale. Beginning with the Mexican Revolution of 1910, with its opposing factions and uncertain outcomes, he attempts to address the great questions of progress, revolution and modernity. Laura's Spanish lover, Jorge, eschewing the recommendations of Marx and Rimbaud, gives this little speech: "We have to diversify life. We have to pluralize the world. We have to give up the romantic illusion that humanity will be happy only if it recovers its lost unity. We have to give up the illusion of totality. The word says it all: there's only a slight difference between the desire for totality and totalitarian reality."

It is in this deceptively subversive truth that Fuentes roots his politically-minded, idealistic characters' dilemmas. If there can't be unity, there can't be revolution. If there can't be revolution, there can't be revolutionary heroes. And where does that leave Jorge's comrade Baltazar, who sees his lover Pilar on the verge of execution by her father, ready to be martyred for Franco? Or Jorge's first lover Raquel, refusing to be rescued from Buchenwald in order to share the fate of her fellow Jews? In a sly plot twist, Pilar is pulled out of the hat late in the day, spirited away by her father at the last moment, and thus deprived of her incipient martyrdom.

Fuentes's project, like that of his cautious radicals, is one of pluralisation. Here, he tries to show us life as a Mexican mural - a recurrent motif throughout the novel - in which different strands, themes and characters can be followed without jeopardising the impact of the whole. It's a good theory, but all too often it results in a literary carelessness, a painful squandering of ideas and people. Two of Laura's aged aunts, for example, end their days by wandering off into the desert to die, writing themselves out of history and refusing to let their final lines be heard. It's an arresting moment, but a fleeting one in a novel that has places to go and people to see.

Amidst the reportage, the history lessons, the showcase love affairs and the earnest self-examination, the ordinary life of the individual struggles to find its place. The result is an epic novel that, for all its length and density, has a peculiarly hasty feel, and for all its populousness, seems curiously empty of real people.